In the specific case of Mastodon, an instance pretty much only receives a post via federation if one of its users either follows the creator of that post, or is mentioned in it.
Discoverability suffers, because this also applies to replies to a post even if you follow its poster. You might see them, or you might not. You look at the post history of one of the users in a thread and it comes up empty.
This is not much of a problem if you’re in one of the, say, top five instances, but beyond that, many functions become increasingly unreliable. Instead of one big microblogging ocean, it feels more like an assortment of a few lakes and myriad puddles with only tenuous interconnection.
Personally, I’ve kinda given up on finding (or creating) my One True Instance and am resorting to having profiles on all of the biggest instances. This also has the advantage that arbitrary defederation decisions affect me to a much lesser extent.
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Apart from that, some payment methods (like crypto) allow transmitting arbitrary amounts. At least, paying for years in advance works without issue. You could pay a few cents and try it out, but be mindful of fees.